(474g) Conceptual Design and Exergy Analysis of the Cryogenic Energy Storage System Integrated with LNG Cold Utilization

According to the Outlook for Energy (ExxonMobil, 2016), liquefied natural gas (LNG) trade is expected to almost triple in the next three decades.¹ Natural gas is transported in liquid state for long-distance because the volume of LNG is approximately 600 times smaller than that of the same mass of gas phase natural gas.² LNG must be re-gasified before it is transported to end users.³ Generally, the cold exergy of LNG is wasted to seawater during the regasification process.⁴ Thus, recovering the cold exergy of LNG is one of the most important issue in LNG regasification step.⁵

This study focuses on the development of an efficient cryogenic energy storage (CES) process using the cold exergy from liquefied natural gas (LNG) regasification. While LNG has low internal energy, it has high exergy because of its cryogenic characteristics, and much of this exergy is wasted in the regasification process.⁶ Thus, this work focuses on the recovery of LNG cold exergy to store cryogenic energy using air as a working fluid. The power generation process by LNG direct expansion requires pumping work to increase the pressure, heat for the LNG vaporization, and the power is generated by an expansion step. The available energy forms produced by LNG regasification process are cold and shaft work. On the other hand, a CES process using air as the working fluid requires shaft work to compress the air and cold to liquefy it. Therefore, the supply and demand of the LNG regasification power generation process and the CES process are well aligned. In this study, we discuss the integration of shaft work and cold to LNG regasification power generation process with direct expansion and a CES process using air. The cold of LNG is transferred in two forms: cold transfer by heat exchange to liquefy air, and shaft work transfer by direct expansion of LNG to compress the air. Thermodynamic analysis of the proposed combined CES and LNG regasification process is carried out in three exergy flow steps: the LNG regasification step, the air liquefaction step, and the air expansion step. Based on exergy analysis, the exergy efficiency is calculated to be 54.4, 94.2, and 61.1 %, for the LNG regasification, air liquefaction, and liquid air expansion steps, and 32.6 % for the overall proposed process. The net amount of work produced by CES system is 67.44 kJ/kg-LNG with 93.48 kJ/kg-LNG of storage work and 160.92 kJ/kg-LNG of work produced by air release process. In addition, the proposed system has an advantage which system can storage and release the energy simultaneously. Therefore, daily produced energy per 1 kg of LNG by CES system is more than double compare to the most recent contributions with storage and release time divided systems. This study not only proposes an efficient energy storage process that can generate power flexibly but also highlights further possibilities for performance enhancement by thermodynamic analysis.

REFERENCES

[1] ExxonMobil, “The Outlook for Energy: A view to 2040,” (2016).

[2] Lee, I., et al., “Design and optimization of a pure refrigerant cycle for natural gas liquefaction with subcooling.,” Ind. Eng. Chem. Res., 53 (25), 10397-403 (2014).

[3] Lee, I., et al., “Decision Making on Liquefaction Ratio for Minimizing Specific Energy in a LNG Pilot Plant,” Ind. Eng. Chem. Res., 54 (51), 12920-12927 (2015).

[4] Szargut. J., and Szczygiel. I., “Utilization of the cryogenic exergy of liquid natural gas (LNG) for the production of electricity,” Energy 34 (7), 827-837 (2009).

[5] Gómez. M.R., et al., “Review of thermal cycles exploiting the exergy of liquefied natural gas in the regasification process,” Renew. Sust. Energy. Rev., 38, 781-795 (2014).

[6] Lee, I., et al., “Conceptual design and exergy analysis of combined cryogenic energy storage and LNG regasification processes: Cold and power integration,” Energy, 140, 106-115 (2017).